The present invention relates to a DTMF (Dual-Tone Multi-Frequency) tone signal transmission method and a DTMF tone signal transmission system used to transmit a DTMF tone signal. In particular, the present invention relates to a DTMF tone signal transmission method and a DTMF tone signal transmission system used to transmit a DTMF tone signal originated from an IP (Internet Protocol) telephone terminal.
Previously, a push-phone can transmit a DTMF tone signal. Such a DTMF tone signal is used not only to transmit a telephone number but also to receive facsimile service or transmit a numeral or the like (“0,” “1,” “2,” . . . “9,” “*” or “#”) in accordance with a guide of an answer-phone.
On the other hand, the IP telephone has been spread in recent years. The IP telephone uses the TCP/IP (Transmission control Protocol/Internet Protocol) as the third layer and the fourth layer of the OSI reference model, uses the SIP (Session Initiation Protocol) to conduct signaling in its upper layer, and uses the RTP (Real-time Transport Protocol) to conduct transmission of a voice codec signal. Not only the IP telephone terminal, but also fixed telephone connected to a PSTN (Public Switched Telephone Network) or a portable telephone connected to a portable telephone network becomes the opposite party of the IP telephone terminal.
As preceding technique papers relating to the present invention, Japanese Patent Application Laid-Open Publication Nos. 2000-059471, 2000-188611, 2003-219027 and 2004-064408 can be mentioned.
The IP telephone terminal is connected to a fixed telephone connected to a PSTN or a portable telephone connected to a portable telephone terminal, as well. Therefore, it is necessary for a DTMF tone signal associated with a numeral or a symbol of a button depressed by the user of the IP telephone terminal to arrive at the fixed telephone or portable telephone of the opposite party.
However, IP telephone terminals do not always have a function of sending out a DTMF tone signal.
In the case where a plurality of IP telephone terminals are connected to a local area network, it increases the cost to provide every IP telephone terminal with a function of sending out a DTMF tone signal.